This study involves the characterization of the primate polyomaviruses BK, JC, SV40, STMV, and SA12. In addition, zur Hausen has recently identified a lymphotrophic polyomavirus in African Green monkeys and serologic evidence suggests that a human lymphotrophic virus also exists. BK, JC, and SV40 have each been shown to be oncogenic in hamsters and each has been demonstrated capable of transforming certain rodent cells in tissue culture. While BK virus has not been clearly associated with any human disease, JC virus has repeatedly been isolated from diseased brains of patients with progressive multifocal leukoencephalopathy (PML). The purpose of this work is to better characterize the polyomaviruses in terms of their biologic and molecular characteristics and to characterize the biologic features of polyomavirus transformed cells. An understanding of how these viruses interact with human cells will provide a better understanding of what pathogenic roles may be involved in any human disease associated with these viruses.